Polyolefins, such as polypropylene, have been used in a variety of applications which require good stiffness and flow properties. However, modifications to the polyolefin composition have been required in order to overcome various disadvantages such as impact resistance, stress whitening, heat resistance, dimensional stability, and mold shrinkage. Other polymers have been blended with polyolefins to try to improve such properties.
Many different polymers and materials have been added to polyolefins to enhance the impact strength of the overall composition. For example, U.S. Pat. No. 5,118,753 (Hikasa et. al.), incorporated herein by reference, discloses thermoplastic elastomer compositions said to have low hardness and excellent flexibility and mechanical properties, consisting essentially of a mixture of an oil-extended olefinic copolyer rubber and an oletinic plastic, such as propylene. Modern Plastics Encyclopedia/89 mid Oct. 1988 Issue, Volume 65, Number 11, pp. 110-117, also discusses the use of various thermoplastic elastomers (TPEs) useful for impact modification, including elastomeric alloys TPEs engineering TPEs, olefinic TPEs (also known as thermoplastic olefins or TPOs), polyurethane TPEs and styrenic TPEs.
Thermoplastic olefins (TPOs) are generally produced from compositions of an elastomeric material such as ethylene/propylene rubber (EPM) or ethylene/propylene diene monomer terpolymer (EPDM) and a more rigid material such as isotactic polypropylene.
Flexomer(trademark)Polyolefins by Union Carbide were introduced to replace expensive EPM or EPDM rubbers. These new polyolefins are said to have bridged the gap between rubbers and polyethylene, having moduli between the two ranges. However, according to the data contained in FIG. 4 of the paper xe2x80x9cFlexomer(trademark) Polyolefins: A Bridge Between Polyethylene and Rubbersxe2x80x9d by M. R. Rifi, H. K. Ficker and M. A. Corwin, more of the Flexomer(trademark) Polyolefin needs to be added into the TPO formulation in order to reach the same levels of low temperature Gardner Impact performance as the standard EPM rubber, thus somewhat negating the benefits of the lower cost EPM/EPDM replacement.
In a paper presented on Sep. 24, 1991 at the 1991 Specialty Polyolefins Conference (SPO ""91) (pp. 43-55) in Houston, Tex., Michael P. Jeffries (Exxpol Ethylene Polymers Venture Manager of Exxon Chemical Company) also reports that Exxon""s Exact(trademark) polymers and Plastomers can be blended into polypropylene for impact modification. Exxon Chemical Company, in the Preprints of Polyolefins VII International Conference, page 45-66. Feb. 24-27 1991, also disclose that the narrow molecular weight distribution (NMWD) resins produced by their EXXPOL(trademark) technology have higher melt viscosity and lower melt strength than conventional Ziegler resins at the same melt index. In another recent publication, Exxon Chemical Company has also taught that NMWD polymers made using a single site catalyst create the potential for melt fracture (xe2x80x9cNew Specialty Linear Polymers (SLP) For Power Cables,xe2x80x9d by Monica Hendewerk and Lawrence Spenadel, presented at IEEE meeting in Dallas, Tex., Sep., 1991).
It is well known that narrow molecular weight distribution linear polymers disadvantageously have low shear sensitivity or low I10/I2 value, which limits the extrudability of such polymers. Additionally, such polymers possessed low melt elasticity, causing problems in melt fabrication such as film forming processes or blow molding processes (e.g., sustaining a bubble in the blown film process, or sag in the blow molding process etc.). Finally, such resins also experienced surface melt fracture properties at relatively low extrusion rates thereby processing unacceptably and causing surface irregularities in the finished product.
Thus, while the development of new lower modulus polymers has aided the TPO marketplace, there continues to be a need for other more advanced, cost-effective polymers for compounding into polypropylene which improve or maintain low temperature impact performance and modulus.
In addition, compatibility of added polymers with the polyolefin is a potential problem. Hivalloyxc2x7 by Montell attempts to expand the performance window of polyolefin blends, wherein polypropylene is grafted with a functional group to aid in compatibility of the two polymers. However, grafting is an additional step, adding cost to the process.
Vinylcyclohexane polymers have also been previously combined with polyolefins such as polypropylene. The use of crystalline vinyl cyclohexane polymer as a nucleating agent for the crystallization of polypropylene is known in the art. However, the amount of vinyl cyclohexane polymer is not more than 0.1 weight percent and does not significantly enhance the properties of the polypropylene. JP-5271482, assigned to Mitsubishi Kasei, discloses a blend of a crystalline polyolefin, such as polypropylene and an amorphous polyvinylcyclohexane type resin in ratios of 95/5 to 5/95. However, the composition still suffers from insufficient impact properties.
Therefore, there remains a need for a polyolefin composition, particularly a polypropylene composition having improved balance of flow, modulus and impact resistance properties which is cost effective and efficient, without the disadvantages of the prior art.
Polyolefin compositions have now been produced, having a combination of good low temperature impact performance and modulus, wherein the composition comprises:
A) a polyolefin thermoplastic,
B) a hydrogenated block copolymer of a vinyl aromatic and conjugated diene monomer having a level of aromatic hydrogenation of at least 70 percent, and
C) at least one linear or substantially linear ethylene/xcex1-olefin polymer,
This composition offers improved flow, stiffness and impact resistance property balance over olefinic polymer compositions of the art.